kbuild: support include/generated

We need a location for generated files.
Today they are spread over several places and bringing them
together to a common place makes it obvious hat is generated
and what isreal files.

Al Viro originally suggested: include/gen
Linus suggested to spell it out.

This patch implement support for

    include/generated

All files in include/generated are ignored by git.
include/generated is removed during "make mrproper".

With this we are ready to implement support for include/generated
in the various architctures and in the base kernel.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>